(1) Field of the Invention
The invention pertains to a shell-and-tube heat exchanger (THE) which contains a wear-resistant tube plate lining for application in thermal cracking equipment.
(2) Description of Related Art
Shell-and-tube heat exchangers of this type are used, for example, in ethylene equipment to produce ethylene through thermal cracking downstream from the transfer line of a cracking furnace and are referred to as quench coolers (transfer line exchangers, or TLE's).
Quench coolers must conform to unusually high standards of construction and material characteristics. The hot reaction mixture discharged from a cracking furnace after pyrolysis of hydrocarbon materials such as naphtha, LPG, ethane, or even hydrocracking residue (unconverted oil, waxy), which can reach temperatures of approximately 850° C., must be cooled quickly in the quench coolers in order to avoid undesirable side reactions. The quench cooler, or THE, functions as a waste heat boiler in which high steam pressure can be created through evaporation of feed water introduced to the casing side.
Coking retardation occurs in cracking furnaces during this process, which must be removed at specific intervals (60-80 days) through oxidation with air. In order to remove the coking, the furnace is heated to minimal operating levels and a mixture of air and steam is introduced into the tubes of the cracking furnace. The carbon residues are burned off with this mixture. Particles of coking are loosened at the same time, and they are carried with the gasses along the cracked gas pathways through the quench cooler and into the coking removal conduits.
The cracked gas or coking removal gas, which is discharged at high velocity from the cracking furnace, generally crosses a transfer line into an axial gas entry chamber and then, from below, into the quench cooler, where it collides against the lower tube plates before it is fed into the remainder of the process after its journey through the heat exchanging tubes of the quench cooler.
In spite of short exposure periods, the cracking gas contains coking particles which become highly corrosive at the high velocities reached by the cracking gas. In order to cool the hot reaction mixture created in the cracking furnace quickly, the distance between cracking furnace and the cooling tubes must be traversed as quickly as possible. This necessitates that the gas entry chamber design be compressed, which would normally broaden out in diameter of the transfer line leading to the cooler, with the result that the stream of gas containing coking particulate is concentrated on the middle region of the tube plate and the cooling tubes, which are affected particularly severely. The weight-bearing wall elements are weakened, which creates the necessity of significant maintenance costs, and maintenance downtimes result in production downtimes.
Various solutions have been proposed to this and similar problems. These are based on using of ceramic, fireproof materials as linings, structural members, or emulsions:
EP-A-0 567 674 introduces heat exchangers for the cooling of synthetic gas created in coal gasification equipment. In this application, the tube plate on the gas inlet side is covered by cuboid-based nozzles which are positioned adjacent to and abutting one another on the outer edges. Each of these nozzles has a conical opening which narrows to a section of tubing which in turn is inserted into a heat exchanger tube. This solution offers no gastight closure between the individual cuboid-based elements. This would lead to buildup of coking residue in the empty spaces within the quench coolers of an olefin processor and destroy the materials. In addition, the ends of the nozzles which are used would form a tearing edge within the tube which, considering the high flow velocities within the quench coolers, would result in heavy turbulences. This would result in additional erosion.
In DE-C-44 04068 a ceramic lining is revealed, which is comprised of fireproof molded elements. These can in some instances be hexagonal in shape and contain perforations through which pins or hooks can be inserted which are welded to the underside of the tube plate. The molded element can be attached to the tube plate in this fashion. This construction does not accomplish the goal of having a seamless emulsion or lamination.
In addition, it is widely accepted that a cooling tube installed within a reactor must be equipped with a fireproof lining which is resistant to erosion (see U.S. Pat. No. 4,124,068), in order to reduce the risk of tube failure and the resulting penetration of cooling water into the surrounding reaction mixture at elevated temperatures.
The suggestion is made in DE 195 34 823 A 1 that tube plates on the gas inlet side be coated or lined with a chemically hardened, erosion-resistant, fireproof product. This coating should initially be formed from a castable substance, applied in a pliable format, and finally fired to achieve its final format as a fireproof mass.
What these applications have in common is that they all combine ceramic—in other words, non-metallic—materials with the metallic apparatus material, principally steel. Practical experience has taught that combining ceramic and metallic components results in increased time, effort, and expense during manufacture, assembly, and repair and often creates problems because of the differences in material characteristics such as thermal expansion coefficients and varying levels of elasticity (brittle vs. ductile). In addition to these issues, inserted ceramic nozzles also have the problem of turbulence and the accompanying particular stresses to the materials at the rear end of the nozzle (from the perspective of flow direction) associated with the tearing edge located there. Contrary to the design laid out in DE 195 34 823, installing a lining consisting of a fireproof mass positioned solely in the focal area in the center of the tube plate has been shown to be impractical because the resulting inconsistent surface of the tube plate, in conjunction with the variations in material characteristics, results in the occurrence of particular problems in the transition range, i.e., at the outer edge of the fireproof form, possibly through chipping or spalling of the form or through particularly heavy erosion caused by turbulence at the edge. It should also be noted that installation of a lining made of a fireproof material only protects the tube plate as such. There is a distinct advantage to providing protection for at least the front part of each cooling tube (viewed from the perspective of flow direction). This can only be accomplished by applying a protective nozzle or sleeve.
The problem of a significantly stronger flow rate and stress on the focal area as compared to the peripheral areas was also investigated. One proposed solution incorporated cone-shaped fittings (see U.S. Pat. No. 3,552,487). Another proposed installing diffuser-type diversion fixtures which could be free-standing within the entry chamber (see DE-PS 21 60 372).
For the purposes of equalizing the flow through the entry chamber and also protecting the tube plate from erosion, an additional suggestion has been made that the THE be fitted out with inserted elements made of small rods bent into the shape of rings. The rings would be aligned along the surface of a cone, and the point of the cone would be directed toward the gas inlet (see EP 037 089 A1).
The intention here is that the coking particles carried along in the focal range of the high-velocity gas flow would be decelerated and in part deflected outward in a radial pattern, so that they would then no longer contribute to erosion damage on the tube plate and the tubes. However, an undesirable differential pressure and the associated increased exposure time would result with fittings of this type, meaning that there would be a yield loss.